When evaluating an earth formation, a core sample from the earth formation may be procured using a bottom hole assembly often referred to in the art as a “coring tool.” A coring tool may include a core bit, which is often a hollow earth-boring rotary drill bit having a longitudinal aperture extending through the center thereof. As a result, when the core bit drills through the formation, a core sample is formed within the longitudinal aperture extending through the center of the core bit. An inner barrel may then be positioned within an outer tubular member, commonly termed a “core barrel” of the coring tool above the core bit, and is configured and positioned to receive the core sample therein as the core sample is formed by the core bit as the core bit drills into the earth formation and the coring tool lowers around the core sample.
During a coring operation, as the core sample is being formed by the core bit and the inner barrel progressively slides downward over the core sample within the coring tool, the core sample may jam rotationally, longitudinally, or both inside the inner barrel. Continued drilling by the core bit when the core sample has jammed inside the inner barrel often results in damage to the core sample, and information regarding characteristics of the earth formation being cored that might otherwise have been obtained from the damaged portion of the core sample is lost.
In an effort to mitigate the effects of such core jams, tools have been developed for use in conjunction with, or as part of, a coring tool that indicate to an operator of the coring tool at the surface of the formation that a core jam has occurred, which allows the operator to attempt to address the issue without causing further damage to the core sample. Some such core jam indicators are mechanical core jam indicators that provide a signal to the operator in the form of an increase in the hydraulic standpipe pressure within the drill string above the coring tool. For example, some previously known mechanical core jam indicators rely on mechanical movement of parts within the core jam indicator induced by a jam between the core sample and the inner barrel. The mechanical movement of parts causes a restriction in a flow area through which hydraulic fluid (e.g., drilling mud) flowing through the tool during the coring operation may pass. The restriction in the flow area results in an increase in the hydraulic standpipe pressure, which is detected by the operator to indicate the presence of the core jam.
Previously known mechanical core jam indicators, however, often require relatively high weight-on-bit for proper operation and, thus, were not usable in some coring operations due to the inability to provide sufficient weight-on-bit. In addition, in previously known mechanical core jam indicators, the increase in the standpipe pressure caused by the core jam indicator responsive to a core jam resulted in application of undesirable hydraulic forces to components of the core jam indicator, which tended to counteract the movement of the mechanical components of the core jam indicator. As a result, a weight-on-bit sufficient to allow initiation of movement of the components of the core jam indicator might not be sufficient to result in complete movement of the components and generation of the pressure change signal in the hydraulic standpipe pressure. This is especially the case in applications where it might be desirable to apply only a limited amount of weight-on-bit.